1. Field of the Invention
The present invention relates to a cap structure for a pulley-fixing nut of a vehicle-use electric rotating machine mounted on a vehicle such as a passenger car or a truck
2. Description of Related Art
Generally, a vehicle-use electric rotating machine, which is mounted on a vehicle to be driven through a belt or to drive various auxiliaries through a belt, includes a pulley for transmitting a driving force between the belt and the rotor of the vehicle-use electric rotating machine. The pulley is fixed to an end of the rotating shaft of the rotor by tightening a nut. It is preferable to rust-proof the end of the rotating shaft after the pulley is fixed using the nut to improve marketability of the vehicle. However, plating or antirust-coating the end of the rotating shaft after tightening the nut increases the manufacturing cost substantially.
It is known to put a cap on the end of the rotating shaft after tightening the nut so that the end of the rotating shaft becomes resistant to rust, or at least rust which has occurred on the end of the rotating shaft becomes invisible. For example, refer to Japanese Utility Model Application Laid-open No. S57-172920. This Application describes that a rust-preventing cap, which covers both the outer periphery of such a nut and the end of a bolt engaging with the nut, is fixed by fitting projections formed in the inner periphery of the cap to the thread portion of the bolt.
However, using the rust-preventing cap involves a problem that, in a case where the bolt end portion protruding from the nut is not sufficiently long, it is not possible to sufficiently fit the projections formed in the inner periphery of the cap to the thread portion of the bolt. In such a case, if a longer bolt is used so that the bolt end portion protruding from the nut is sufficiently long, it is possible to fix the rust-preventing cap. However, in this case, it becomes difficult to reduce the size of the vehicle-use electric rotating machine including the rust-preventing cap, and the size of the rust-preventing cap itself has to be increased.
In addition, since the rotating shaft of the vehicle-use electric rotating machine rotates at a high speed and accordingly the nut also rotates at the high speed, there is a concern that the rust-preventing cap may come off because of decrease of the fixing force between the projections of the rust-preventing cap and the thread portion of the bolt, which is caused by the centrifugal force, a force in the rotational direction caused by variation of the rotational speed, and environmental stress (high vibration and high temperature).